Scanning-electron beam lithography uses an electron beam to write patterns, e.g. mask patterns, in electron sensitive films on substrates. An electron beam can be focused to a diameter of less than 10 nm, allowing patterns of extremely fine dimensions to be written.
For maskless lithography, electron beams would appear to offer the ideal solution, if only two problems could be solved: slow speed and poor pattern-placement accuracy.
When an electron beam is focused to a fine spot, patterns are written in a pixel-by-pixel manner. This pixel-by-pixel addressing is slow for an electron beam system. To address this problem, it has been proposed to project several pixels in parallel, i.e., the shaped beam approach.
Conventional methods using shaped-beam electron projection suffer from three shortcomings. First, the current density has to be reduced by orders-of-magnitude relative to the current density in a “round-beam” system in order to limit the effects of mutual electrostatic repulsion. Thus, if 1000 pixels are projected in parallel, the throughput does not go up by a factor of 1000 but rather only by a factor of 10 or less.
Second, pattern-placement accuracy and pattern distortion become very serious limitations in shaped-beam systems. To address this issue, “product-specific emulation” schemes are conventionally employed, but these are not stable in time. The end result is that shaped beam systems have to operate at very high voltages and are highly expensive.
Third, the shaped-beam approach assumes that the patterns consist of elements containing hundreds or thousands of pixels. However, this is not always the case. Conventionally, the most demanding patterns are generally composed of dense features at minimum linewidths (e.g., DRAM patterns). In this case the utility of the shaped-beam approach is highly limited.
As noted above, pattern-placement inaccuracy is a persistent problem in energy-beam lithography techniques such as scanning-electron-beam lithography. Pattern-placement errors stem from a variety of environmental and system variations; however, the fundamental issue is the open-loop nature of these systems, i.e. the beam location on the substrate is not monitored during exposure.
However, the energy-beam locating method described in U.S. Pat. No. 5,136,169 can be used to provide closed-loop control of the beam position by monitoring the signal from a fiducial grid on the substrate. The entire content of U.S. Pat. No. 5,136,169 is hereby incorporated by reference.
One conventional solution for determining the location of an energy beam in two dimensions that utilizes a fiducial grid on the substrate requires that the fiducial grid have two different spatial-periods. This conventional approach sacrifices performance because one period is necessarily coarser than the other.
One conventional solution for determining the location of an energy beam in two dimensions that utilizes a fiducial grid on the substrate requires a fiducial grid with two different signal carriers (e.g. two different optical emission wavelengths). This second conventional approach adds undesirable complexity to the system and to the grid itself.
Therefore, it is desirable to provide a scanning-electron beam lithography that avoids pattern-placement inaccuracy. Moreover, it is desirable to provide a scanning-electron beam lithography which is capable of determining energy beam location in two-dimensions.
Furthermore, it is desirable to provide a scanning-electron beam lithography that is quicker than conventional systems. It is also desirable to provide a scanning-electron beam lithography that utilizes multiple parallel energy beams. Lastly, it is desirable to provide a scanning-electron beam lithography that determines the location of several parallel energy beams and the beams' shape.